The invention described and claimed herein relates to an electrochemical method for separating a metal from a salt thereof and an apparatus for performing the same. In particular, this invention relates to an improved method and apparatus of the type wherein a metal is separated from an electrically dissociable molten salt thereof by electrically dissociating the cations of the metal and the salt from the anions of the salt, passing the cations through a solid electrolyte that is selectively conductive to the cations and impermeable to the anions, and reducing the cations to elemental metal. U.S. Pat. No. 3,488,271 issued Jan. 6, 1970 to Kummer et al discloses and claims such a method for separating a metal from a salt.
Although the method disclosed and claimed in the Kummer et al patent is effective for separating metal from its electrically dissociable salt, the method and the apparatus for performing the method suffer from certain disadvantages relating to the degradation of the solid electrolyte, most commonly a beta-type alumina ceramic. Although the mechanism by which the solid electrolyte degrades, thereby shortening the life of the apparatus and decreasing the rate of metal separation, is not known, it is surmised that the degradation or cracking of the solid electrolyte ceramic results from reduction of the metal cation at the surface of the solid electrolyte. One possibility is that the reduction of the cation occurs in microfissures or cracks in the ceramic causing further cracking of the ceramic to occur or be propagated and resulting in a necessary limitation of allowable current density. A still further possible cause of ceramic degradation and decreased performance could be reduction at the interface of the solid electrolyte of cationic impurities which may be present in the dissociable salt being electrolyzed. Such impurities may be solid at the cell operating temperature, thus resulting in dendritic growth of impurity metals around and through the ceramic.
It is the object of this invention to modify the prior art apparatus and method in such a manner as to reduce or sub stantially eliminate solid electrolyte degradation, thereby increasing the permissible rate of metal separation and the life of the separation cell.